Blade for fluid flow machines

ABSTRACT

The disclosure of this invention relates to a rotor blade for a fan for a gas turbine engine. The blade has in succession an aerofoil, a shank and a root, and the shape of the aerofoil is continued into the shank but at progressively reducing camber so that, whereas adjacent the aerofoil the camber of the shank is the same as that of the aero-foil, adjacent the root the camber of the shank is zero. This leads to a reduction in the stresses produced by a torsion couple arising in operation and acting in the sense tending to reduce the twist of the blade.

[ Mar. 18, 1975 BLADE FOR FLUID FLOW MACHINES [75] Inventors: KennethRonald Guy; Robert Burns Hood, both of Bristol, England [73] Assignee:Rolls-Royce (1971) Limited,

London, England 22 Filed: Feb. 20, 1973 21 App1.No.:333,659

[30] Foreign Application Priority Data Mar. 9. 1972 Great Britain11078/72 [521 US. Cl 416/193, 416/196, 416/234, 416/242 [51] Int. Cl.FOld 5/14, FOld 5/22 [58] Field of Search 416/193, 223, 239, 234,416/242, 248, 196, 191,236 A [56] References Cited UNITED STATES PATENTS1,772,876 8/1930 Parsons et a1..... 416/234 UX 1,981,392 11/1934 Selman416/223 2,193,616 3/1940 Baumann.. 416/236 X 2.327.453 8/1943 Presser416/239 X 2.391.623 12/1945 Heppner i 416/191 2.421.890 6/1947.lohansson 416/193 2,974,728 3/1961 Culp 416/239 2.999.668 9/1961 Howaldet a1 416/97 3.012.709 12/1961 Schne11 416/236v 3,173,490 3/1965 Stuart416/223 3,477,795 11/1969 Beesley 416/193 X 3,490,852 l/l970 Carlstromet a1. 416/95 3,576,377 4/1971 Beanland et a1 416/191 FOREIGN PATENTS ORAPPLlCATIONS 932,045 11/1947 France 416/95 1,263,677 5/1961 France416/193 1,330,690 5/1963 France 416/223 229,266 10/1943 Switzerland416/234 8,524 2/1965 Japan 416/193 1,187,166 3/1959 France 416/236 APrimary ExaminerEverette A. Powell, Jr. Attorney, Agent, orFirm-Stevens, Davis, Miller & Mosher [57] ABSTRACT The disclosure ofthis invention relates to a rotor blade for a fan for a gas turbineengine. The blade has in succession an aerofoil, a shank and a root, andthe shape of the aerofoil is continued into the shank but atprogressively reducing camber so that, whereas adjacent the aerofoil thecamber of the shank is the same as that of the aero-foil, adjacent theroot the camber of the shank is zero. This leads to a reduction in thestresses produced by a torsion couple arising in operation and acting inthe sense tending to reduce the twist of the blade.

4 Claims, 6 Drawing Figures pmmggum 8197-5 9.871. .791

Skill 1 BF 2 BLADE FOR FLUID FLOW MACHINES This invention relates toblades for fluid flow machines.

lt is known for such blades to comprise an aerofoil, a shank and a root.The aerofoil manifests the stagger, camber and cross-sectional shaperequired by the aerodynamics of the blade. The root is a portion of theblade shaped to engage a recess in a rotor body thereby to support theblade inter alia against centrifugal force. The shank is a portion ofthe blade connecting the root to the aerofoil and providing a degree offlexibility between root and aerofoil. However, in known blades theshank is still of substantially greater cross-section than the aerofoilso that the junction of aerofoil and shank is a locality of relativelyhigh stress concentration.

For aerodynamic reasons the stagger of the aerofoil is of progressivelyreducing magnitude in the radially inward direction, this change ofstagger being referred to as twist." Under centrifugal force the twistof the blade gives rise to a torsion couple in the sense tending toreduce the twist. At sections of the aerofoil remote from any endrestraint the torsion couple gives rise to a diminution in thecentrifugal stress at the leading and trailing edges of the aerofoil andan increase in the centrifugal stress at the medial parts of theaerofoil crosssection. The stresses so produced tend to produceoutof-plane deformation of the cross-section. At the junction betweenaerofoil and shank the relatively greater stiffness of the latterrestrains the out-of-plane deformation of the adjacent aerofoil andgives rise to socalled warping restraint stresses. It can be shown thatthese stresses increase with increasing camber.

In relatively highly cambered and twisted rotor blades for aero-enginefans the warping restraint stresses dominate the stress pattern at theinner end of the aerofoil and this can be critical for the design of theblade. It is an object of the invention to provide a construction ofblade in which the warping restraint stresses are reduced.

According to the invention there is provided a blade for a fluid flowmachine, comprising in succession an aerofoil, a shank and a root,wherein the shank has at its radially outer end a stagger, camber andcrosssectional shape similar to that of the adjacent end of theaerofoil, and wherein the shank is shaped for the camber thereof to beprogressively reducing towards the root end of the shank.

The reduction of the camber leads to a corresponding reduction inwarping restraint stresses, the latter now occurring at the root end ofthe shank. Preferably the shank is made sufficiently long to make itpossible to reduce the camber to zero at the root end thereof. Therebythe warping restraint stresses are reduced to a minimum.

The stagger of the shank may also be reduced progressively towards theroot end of the shank to make it possible to improve the stressdistribution in the shank and to reduce stress concentrations at theroot recess in the rotor body.

Examples of blades according to this invention will now be describedwith reference to the accompanying drawings wherein:

FIG. 1 is a side elevation ofa fan for a gas turbine engtne emobodymg ablades according to the invention.

FIG. 2 is a view in the direction of arrow II in FIG. 1.

FIG. 3 is a plan view of FIG. 2.

FIG. 4 is a section on the line C-C in FIG. 1 but having the sameorientation as FIG. 3.

FIG. 5 is a view similar to FIG. 2 but embodying a modification.

FIG. 6 is a view similar to FIG. 4 and embodying the modification shownin FIG. 5.

Referring to FIGS. 1 to 4, there is shown a rotor 10 comprising a disc11 and blades 12 (only one shown). Each blade comprises in succession anaerofoil 13, a shank l4 and a root 15. At the junction between the shankand the aerofoil the blade comprises a platform 16 which is a wall partof the fluid flow passage controlled by the aerofoil. The blade isconnected to the disc by interdigitation (FIG. 2) between the rootand arecess 18 in the disc. The axis of rotation of the rotor is denoted 19(FIGS. 3,4). The direction of rotation of the blade is shown by an arrow20 (FIGS. 2,3). A line 21 is the locus of the centres of gravity ofsuccessive cross-sections of the blade transverse to the line 21.

The cross-sections of the blade at lines AA, B-B, CC, D-D (FIG. 1) aredenoted A1, B1, C1, D1 respectively in FIGS. 2, 3,4.

The change in cross section between the radially inner and outer ends ofthe aerofoil is determined in accordance with known principles and neednot be discussed in detail, but attention is drawn to the change instagger and to the presence of camber.

Taking section A1 as an example (FIG. 2), the stag ger is defined by anangle a between the chord line, 22, of the section and the axis 19. Thecamber is defined as the curvature (i.e., rate of change of slope) ofthe socalled camber line 23A of the section. The shape of the section isgiven by the contour 24A of the section as shown.

The stagger of the aerofoil progressively reduces between the sectionsAl, B1, the stagger of the latter section being given by an angle B. Thechange in stagger defines the twist of the blade. The camber of sectionB1 is greater than that of section A1 and is given by a line 238. Theshape of section BI is given by the contour 248.

In the past it has been the practice to make the crosssection of theshank substantially larger than that of the radially innermost sectionB1 of the aerofoil, and the cross-section of the shank was more nearlythe same as the parallelogrammic shape of the root as shown in FIG. 3.In accordance with the invention, the radially outermost end, i.e., thesection C1, of the shank is formed (FIG. 4) to have a stagger angle B,camber line 23C and cross-sectional shape 24C similar to those of theadjacent end, i.e., the section B], of the aerofoil, and the shank isshaped so that the camber of successive sections thereof is ofprogressively reducing magnitude. As shown in FIG. 4 the camber of theradially in nermost section D1 ofthe shank is zero, i.e., the section issymmetrical about a line 25. The line 25 is also the axis of symmetry ofthe root 15.

As shown in FIGS. 1, 4 a shank shaped in this way has leading andtrailing edges 26 and 27 respectively which extend from the section Cl,where the edges 26, 27 are remote from the plane, referred to as theplane of symmetry of the root 15, of the lines 21, 25 to a position atthe section D1 where the edges 26, 27 intersect that plane. The edges26, 27 are curved to have a common tangent with leading and trailingedges 28 and 29 respectively of the aerofoil so that good continuity ofform is preserved at the junction of aerofoil and shank with a view ofminimising stress concentrations.

Similarity between the sections B1, C l is affected by the angles atwhich the edges 28, 29 enter the platform 16 and by the thickness of theplatform. This results, in this example, in that the section Cl is ofshorter span and of lesser stagger angle that the section B1, and theterm similar applied to thesections B1, C1 is intended to include thesedifferences.

In use, when the blade is subject to centrifugal force, the warpingrestraint stresses occur at the junction of shank and root i.e., at thesection D1. By virtue of the absence of any camber at the section D1 thewarping restraint stresses are therefore only those created by the twistof the blade itself.

It will be noted from FIG. 2 that in order to preserve continuity ofline between the edges 29, 27, the latter has to be curved fairlyacutely, and in the opposite sense to that of the edge 29, in order tofit the condition of having at its one end a common tangent with theedge 29 and at its other end lying on line 25. This acute curvature canbe a disadvantage if the shank is to be relatively short and the twistand camber are relatively high. This situation can be avoided bychanging the stagger of the root in the sense of reducing it, i.e.,bringing the root more nearly into alignment with the axis 19.

The result of such a change in root stagger are shown in FIGS. 5 and 6which show a blade in which the stagger of the root is zero, i.e. theroot has a plane of symmetry aligned to the axis 19 (FIG. 6). Thecurvature of the edges 26,27 is now more nearly the same, the curvatureof the trailing edge 27 being less acute at the expense of the curvatureof the leading edge 26 being tolerably more acute. This results not onlyin a better stress balance in the shank but also makes it possible forthe recess 18 in the disc to be parallel to the axis 19. This simplifiesthe machining of the recess and avoids the stress concentrationsencountered in recesses having a pronounced stagger.

We claim:

I. A fan rotor for gas turbine engines comprising:

a disc having an axis of rotation,

fan blades connected to the disc and extending radially from theperiphery thereof,

means defining a wall extending circumferentially about said axis inspaced apart relationship to the periphery of the disc,

each of said blades comprising an aerofoil of given camber and twistextending radially outwardly of said wall and having a radially innerend adjacent said wall, the chord of said aerofoil generally increasingradially outwardly,

a shank extending between the inner end of the aerofoil and a locationadjacent the periphery of the disc, said shank having a radially outerend of a cross-section similar in stagger, camber and aerofoil shape tothat of the inner end of the aerofoil, and the camber of the shankgradually progressively decreasing from the outer end to the inner endthereof so as to be zero at the inner end of the shank,

and means provided at the radially inner end of the shank for connectingthe blade to the disc.

2. A fan rotor for gas turbine engines according to claim 1, saidconnecting means comprising a root portion directly adjacent theradially inner end of the shank and radially inwardly thereof, and thedisc including, with respect to each root portion, a recess engageableby the root portion to connect the blade to the disc.

3. A fan rotor for gas turbine engines according to claim 2, wherein theroot portion has a plane of symmetry, the shank and aerofoil each havinga trailing edge, and the trailing edge of the shank, at the radiallyouter end thereof, has a common tangent with the trailing edge of theaerofoil at the radially inner end thereof, and the trailing edge of theshank intersects the plane of symmetry of the root portion at theradially inner end of the shank.

4. A fan rotor for gas turbine engines according to claim 3, the shankand aerofoil each having a leading edge, the leading edge of the shankat the radially outer end thereof has a common tangent with the leadingedge of the aerofoil at the radially inner end thereof, and the leadingedge of the shank intersects the plane of symmetry of the root portionat the radially inner end of the shank.

1. A fan rotor for gas turbine engines comprising: a disc having an axisof rotation, fan blades connected to the disc and extending radiallyfrom the periphery thereof, means defining a wall extendingcircumferentially about said axis in spaced apart relationship to theperiphery of the disc, each of said blades comprising an aerofoil ofgiven camber and twist extending radially outwardly of said wall andhaving a radially inner end adjacent said wall, the chord of saidaerofoil generally increasing radially outwardly, a shank extendingbetween the inner end of the aerofoil and a location adjacent theperiphery of the disc, said shank having a radially outer end of across-section similar in stagger, camber and aerofoil shape to that ofthe inner end of the aerofoil, and the camber of the shank graduallyprogressively decreasing from the outer end to the inner end thereof soas to be zero at the inner end of the shank, and means provided at theradially inner end of the shank for connecting the blade to the disc. 2.A fan rotor for gas turbine engines according to claim 1, saidconnecting means comprising a root portion directly adjacent theradially inner end of the shank and radially inwardly thereof, and thedisc including, with respect to each root portion, a recess engageableby the root poRtion to connect the blade to the disc.
 3. A fan rotor forgas turbine engines according to claim 2, wherein the root portion has aplane of symmetry, the shank and aerofoil each having a trailing edge,and the trailing edge of the shank, at the radially outer end thereof,has a common tangent with the trailing edge of the aerofoil at theradially inner end thereof, and the trailing edge of the shankintersects the plane of symmetry of the root portion at the radiallyinner end of the shank.
 4. A fan rotor for gas turbine engines accordingto claim 3, the shank and aerofoil each having a leading edge, theleading edge of the shank at the radially outer end thereof has a commontangent with the leading edge of the aerofoil at the radially inner endthereof, and the leading edge of the shank intersects the plane ofsymmetry of the root portion at the radially inner end of the shank.